The fabrication of various solid state devices requires the use of planar substrates, or semiconductor wafers, on which integrated circuits are fabricated. The final number, or yield, of functional integrated circuits on a wafer at the end of the IC fabrication process is of utmost importance to semiconductor manufacturers, and increasing the yield of circuits on the wafer is the main goal of semiconductor fabrication. After packaging, the circuits on the wafers are tested, wherein non-functional dies are marked using an inking process and the functional dies on the wafer are separated and sold. IC fabricators increase the yield of dies on a wafer by exploiting economies of scale. Over 1000 dies may be formed on a single wafer which measures from six to twelve inches in diameter.
Various processing steps are used to fabricate integrated circuits on a semiconductor wafer. These steps include deposition of a conducting layer on the silicon wafer substrate; formation of a photoresist or other mask such as titanium oxide or silicon oxide, in the form of the desired metal interconnection pattern, using standard lithographic or photolithographic techniques; subjecting the wafer substrate to a dry etching process to remove the conducting layer from the areas not covered by the mask, thereby etching the conducting layer in the form of the masked pattern on the substrate; removing or stripping the mask layer from the substrate typically using reactive plasma and chlorine gas, thereby exposing the top surface of the conductive interconnect layer; and cooling and drying the wafer substrate by applying water and nitrogen gas to the wafer substrate.
During the photolithography step of semiconductor production, light energy is applied through a reticle mask onto the photoresist material previously deposited on the wafer to define circuit patterns which will be etched in a subsequent processing step to define the circuits on the wafer. A reticle is a transparent plate patterned with a circuit image to be formed in the photoresist coating on the wafer. A reticle contains the circuit pattern image for only a few of the die on a wafer, such as four die, for example, and thus, must be stepped and repeated across the entire surface of the wafer. In contrast, a photomask, or mask, includes the circuit pattern image for all of the die on a wafer and requires only one exposure to transfer the circuit pattern image for all of the dies to the wafer.
The circuit features on a reticle must be precisely fabricated since these features are transferred to the wafer to define the pattern of the circuits to be fabricated on the wafer. Thus, the quality of the reticle is important to produce high-quality imaging during submicron photolithography. If circuit pattern defects such as distortion and incorrect image placement on the reticle are not detected prior to the exposure step, these defects will be reproduced in the resist on the wafer. For this reason, once they are fabricated reticles are typically subjected to extensive automated testing for defects and particles.
Reticles are used in stepper systems and in step-and-scan systems, or scanners, which use a reduction lens to reduce overlay accuracy during circuit patterning. Steppers typically operate under a reduction ratio of 5:1 or 4:1, whereas scanners typically operate under a reduction ratio of 4:1. The small field exposure size on steppers and scanners facilitates precise control of tolerances during reticle alignment.
Steppers and scanners typically include a computer-controlled automatic alignment system which identifies alignment marks on the reticle. The reticle is mounted in a reticle stage and the wafer is supported on a wafer chuck provided on a wafer stage. An illumination system projects light through the alignment marks on the reticle and onto the wafer surface, respectively. Light detectors then optically detect the alignment marks on the reticle and marks on the wafer that are illuminated by the light. Laser infraredometry is used to measure the position of the wafer stage that holds the wafer chuck. Once obtained, the position data is fed into the system computer with a software interface to the electromechanical system used to facilitate the adjustments needed to properly align the wafer to the reticle.
Patterning of the circuit pattern on the photoresist is one of the main factors that dictates product success or failure, and a number of factors can contribute to pattern instability. It has been found that a major source of pattern instability is heat from the light source in the stepper or scanner. This heat raises the temperature of the reticle on the order of 4˜6 degrees Celsius and causes the reticle to expand, thus distorting the circuit pattern in the reticle. Experiments have shown that continuous 10-hour exposure of a reticle (exposure dose=350 mJ/cm2) in an I-line scanner causes the temperature of the reticle to rise from 22 degrees Celsius to 28 degrees Celsius. Furthermore, it has been shown that continuous 10 hour exposure of a reticle (exposure dose=70 mJ/cm2) in a deep UV scanner to rise from 22 degrees Celsius to 26 degrees Celsius.
In further experiments, a wafer was coated with a DUV (deep ultraviolet) photoresist in a diamond-shaped pattern having a length-to-width ratio of 30:1. In one experiment, the reticle was placed in the wafer stage of a DUV scanner for 10 hours prior to exposure of the wafer through the reticle. It was found that the circuit pattern in the reticle was seriously deformed when transferred to the photoresist on the wafer. In another experiment, a reticle which had not been subjected to a 10 hour exposure to the light source in the scanner was placed in the wafer stage and then the wafer was exposed through the reticle. It was found that the pattern in the reticle, when transferred to the photoresist, was not distorted. Additional experiments carried out in an I-line scanner yielded similar results.
Accordingly, a reticle thermal detector is needed to determine whether a reticle is excessively distorted due to thermal effects prior to exposure of a wafer through the reticle, in order to prevent distorted circuit pattern images from being formed on wafers during photolithography.
An object of the present invention is to provide a novel reticle thermal detector which enhances the quality of circuit pattern images formed on semiconductor wafers.
Another object of the present invention is to provide a novel reticle thermal detector which ensures the integrity of a circuit pattern image prior to photolithographic transfer of the image from a reticle to a wafer.
Still another object of the present invention is to provide a novel reticle thermal detector which prevents distorted circuit pattern images from being transferred from a reticle to a wafer during photolithography.
Yet another object of the present invention is to provide a novel reticle thermal detector which enhances the yield of devices on a wafer.
A still further object of the present invention is to provide a novel reticle thermal detector which may use optical, mechanical or electromechanical means to measure thermal distortion of a reticle prior to exposure of a wafer through the reticle.
Another object of the present invention is to provide a novel reticle thermal detector which may be used in a stepper or scanner to ensure the integrity of circuit pattern images transferred from a reticle to a wafer.
Yet another object of the present invention is to provide a novel method of enhancing the quality of circuit pattern images formed on a wafer during photolithography.
Another object of the present invention is to provide an exposure apparatus which includes an exposure device such as a scanner or stepper and a reticle thermal detector provided in the exposure apparatus for determining whether a reticle is distorted prior to a photolithography process.